Brain Freeze mills three cards, which can help dig through your own deck or feed the Flashback cost of Flash of Insight is unrelated, but more importantly this disruption plan is weaker against a deck with no basic lands since Hermit Druid's single activation mills everything regardless. Since graveyard hate like Tormod's Crypt isn't in our sideboard, we instead focus on tempo answers; Chain of Vapor can bounce Volrath's Shapeshifter or Hermit Druid itself before it activates. Stasis is a strong answer since it denies them the mana to activate Hermit Druid or cast their reanimation spells on the turn they need to combo off. Black Vise is weak here since this combo deck doesn't naturally hold a full hand, making the swap for Brain Freeze marginal value at best — consider skipping this swap if preferred.